Abstract. Co-located collaboration on large vertical screens has become technically feasible, but users are faced with increased effort, or have to wear intrusive personal identifiers. Previous research on co-located collaboration has assumed that all users perform exactly the same task (e.g., moving and resizing photos), or that they negotiate individual actions in turns. However, there is limited user interface software that supports simultaneous performance of individual actions during shared tasks (Fig. 1a). As a remedy, we have introduced multi-touch chords (Fig. 1b) and personal action windows (Fig. 1c) for co-located collaboration on a large multi-touch vertical display. Instead of selecting an item in a fixed menu by reaching for it, users work simultaneously on shared tasks by means of personal action windows, which are triggered by multi-touch chords performed anywhere on the display. In order to evaluate the proposed technique with users, we introduced an experimental task, which stands for the group dynamics that emerge during shared tasks on a large display. A grounded theory analysis of users’ behaviour provided insights into established co-located collaboration topics, such as conflict resolution strategies and space negotiation. The main contribution of this work is the design and implementation of a novel seamless identification and interaction technique that supports diverse multi-touch interactions by multiple users: multi-touch chord interaction along with personal action windows.